This invention relates to microcapsules and to a process for their production. More particularly, this invention relates to encapsulated droplets of a liquid material which is substantially insoluble in water, wherein the encapsulating agent is a shell wall containing disulfide units, thereby forming an environmentally sensitive, variable release wall. Further, this invention relates to the processes for the production of such microcapsules and methods for their use.
The use of microcapsules for the slow or controlled release of liquid, solid and solids dissolved or suspended in solvent is well known in the chemical art, including the pharmaceutical, specialty chemical and agricultural industry. In agriculture, controlled-release techniques have improved the efficiency of herbicides, insecticides, fungicides, bactericides and fertilizers. Non-agricultural uses have included encapsulated dyes, inks, pharmaceuticals, flavoring agents and fragrances.
The wall of the microcapsule are typically porous in nature, releasing the entrapped material to the surrounding medium at a slow or controlled rate by diffusion through the pores of the wall. In addition to providing controlled release, the walls also serve to facilitate the dispersion of water-immiscible liquids into water and water-containing media such as wet soil. Droplets encapsulated in this manner are particularly useful in agriculture, where water from irrigation, rain and water sprays is frequently present.
Various processes for microencapsulating material have been previously developed. These processes can be divided into three categories-physical methods, phase separation and interfacial reaction. In the physical methods category, microcapsule wall material and core particles are physically brought together and the wall material flows around the core particle to form the microcapsule. In the phase separation category, microcapsules are formed by emulsifying or dispersing the core material in an immiscible continuous phase in which the wall material is dissolved and caused to physically separate from the continuous phase, such as by coacervation, and deposit around the core particles. In the interfacial reaction category, microcapsules are formed by emulsifying or dispersing the core material in an immiscible continuous phase and then an interfacial polymerization reaction is caused to take place at the surface of the core particles.
The above processes vary in utility. Physical methods such as spray drying, spray chilling and humidized bed spray coating, have limited utility for the microencapsulation of products because of volatility losses and pollution control problems associated with evaporation of solvent or cooling, and because under most conditions not all of the product is encapsulated nor do all of the polymer particles contain product cores. Phase separation techniques suffer from process control and product loading limitations. It may be difficult to achieve reproducible phase separation conditions, and it is difficult to assure that the phase separated polymer will preferentially wet the core droplets.
Interfacial polymerization reaction methods have proven to be the most suitable processes for use in the agricultural industry for the microencapsulation of pesticides. There are various types of interfacial reaction techniques. In one type, the interfacial condensation polymerization microencapsulation process, two different monomers are brought together at the oil/water interface where they react by condensation to form the microcapsule wall.
In another type, the in situ interfacial condensation polymerization reaction, an organic phase which contains an oil core and one or more prepolymers is prepared. It is then dispersed into a continuous or aqueous phase solution comprising water and a surface-active agent. The organic phase is dispersed as discrete droplets throughout the aqueous phase by means of emulsification, with an interface between the discrete organic phase droplets and the surrounding continuous aqueous phase solution being formed. In situ self-condensation at the interface and curing of the polymers in the organic phase droplets is initiated by heating the emulsion to a temperature between about 20xc2x0 C. to about 100xc2x0 C. The heating occurs for a sufficient period of time to allow substantial completion of in situ condensation of the prepolymers to convert the organic droplets to capsules consisting of solid permeable polymer shells enclosing the organic core materials. Depending upon the type of prepolymer used, an acidifying agent may be required in order to maintain the pH of the emulsion at a range of about 0 to about 4 pH during condensation.
Two types of microcapsules prepared by in situ condensation are found in the art. One type, as exemplified in U.S. Pat. No. 4,285,720, is a polyurea microcapsule which involves the use of at least one polyisocyanate such as polymethylene polyphenylisocyanate (PMPPI) and/or tolylene diisocyanate (TDI) as the prepolymer. In the creation of polyurea microcapsules, the wall-forming reaction is initiated by heating the emulsion to an elevated temperature at which point the isocyanate polymers are hydrolyzed at the interface to form amines, which in turn react with unhydrolyzed polymers to form the polyurea microcapsule wall.
Another type, exemplified in U.S. Pat. Nos. 4,956,129, 5,160,529 and 5,332,584, incorporated herein by reference, is an aminoplast microcapsule wherein the wall-forming prepolymer is an etherified or alkylated amino formaldehyde (aminoplast) resin. The aminoplast microcapsule walls are formed by heating the emulsion while simultaneously adding to the emulsion an acidifying agent in order to maintain the emulsion pH at from about 0 to about 4 pH. The heating and lowering of the pH of the emulsion is maintained for a sufficient amount of time to allow in situ self-condensation and/or cross-linking of the amino resin thereby forming the aminoplast microcapsule wall.
Microcapsules produced by in situ condensation have the benefits of high pesticide loading and low manufacturing costs, as well as a very efficient membrane and no monomer residue remaining in the aqueous phase. Further, such microcapsules are capable of effecting a slow or controlled rate of release of the encapsulated material by its diffusion through the microcapsule shell to the surrounding medium.
These controlled release microcapsules provide longer term efficacy as the encapsulated material is released over a period of time and is available throughout the effective period. In the field of agriculture, this is particularly significant for pesticides or other ingredients which are degraded or decomposed over a relatively short period of time under certain environmental conditions. Use of microencapsulated compositions in these situations provides effective activity of the encapsulated ingredient over a longer period of time, typically several weeks, since it is released into the environment continuously in the amount needed rather than in one large initial dose. Controlled release microencapsulated pesticides are primarily used as preemergence pesticides wherein they are applied to the soil prior to the emergence of vegetation or appearance of insects. By such application, they are available over a period of time to kill or control newly emerged weed species or insects in their larval stages. Microencapsulated insecticides and fungicides can also be used for foliar application.
Microencapsulation of products such as pesticides provide the added benefit of increase in the safety of pesticide handling in that the polymer wall of the microcapsule minimizes the contact by the handler with the active pesticide. Still, there are instances where it is desirable to have the benefits of both the controlled gradual release and quick release of the encapsulated ingredient. Such an instance would be where the microcapsule is ingested by a harmful insect. In such a case, it would be desirable for the microcapsule wall to quickly break down, allowing a fast release of the pesticide into the insect gut. Further, in the instance where the microcapsule is ingested by a beneficial or non-harmful insect, it would be desirable that the microcapsule wall not break down, allowing the insect to survive.
It has been discovered that the wall of microcapsules formed by in situ condensation polymerization reaction similar to that described in U.S. Pat. Nos. 4,956,129, 5,160,529 and 5,332,584 can be modified by the inclusion of disulfide links in the aminoplast wall, or by replacement of the amino resin with compounds capable of forming or having disulfide links. These links serve to enhance the properties of the microcapsule wall such that the material contained within are released either by gradual controlled release or fast triggered release depending upon the environment in which the microcapsule is found.
Those environments include, for agricultural applications, the terrain or vegetation where such microcapsules may be applied. In such an environment, the encapsulated material would be released gradually. The environment may also include the gut of an insect, wherein conditions therein would trigger or cause the disulfide links to cleave, thereby allowing a quick or fast release of the encapsulated material. Accordingly, the encapsulated material may be gradually released across the wall of the microcapsule in an environment that does not induce cleavage of the disulfide links, or the disulfide links may cleave due to conditions in the environment surrounding the microcapsule thereby quickly releasing the encapsulated material.
The process for preparing such microcapsules comprises:
(a) preparing an organic solution or oil phase comprising the material to be encapsulated and the wall-forming material, whereby the wall-forming material is dissolved in the organic phase and comprises one or more cross-linking agents, in which at least one of the cross-linking agents is a polythiol compound and, optionally, an alkylated amino-formaldehyde prepolymer;
(b) creating an emulsion of the organic solution in a continuous phase aqueous solution comprising water, a protective colloid and, optionally, a phase transfer catalyst and/or emulsifier, wherein, the emulsion comprises discrete droplets of the organic solution dispersed throughout the continuous phase aqueous solution, with an interface formed between the discrete droplets of organic solution and the aqueous solution; and
(c) causing in situ condensation and/or formation of disulfide linkages and curing of the wall-forming material in the organic solution of the discrete droplets at the interface with the aqueous solution by heating the emulsion and, optionally, simultaneously adding to the emulsion an acidifying agent whereby the pH of the emulsion is maintained between about 0 and about 4 for a sufficient period of time to allow substantial completion of wall formation, thereby converting the organic solution droplets to capsules consisting of solid permeable polymer shells enclosing the material.
Microcapsules formed by this process are capable of effecting a gradual controlled rate of release of the encapsulated material by diffusion through the shell to the surrounding medium. Further, microcapsules formed by this process are capable of effecting a fast rate of release of the encapsulated material by cleavage of the disulfide linkages in the presence of a surrounding medium which would promote such cleavage. The present invention resides in both the process described above and the microcapsules thus formed.
The release rate by Fickian diffusion of an active ingredient from a microcapsule may be defined by the equation:   release_rate  =                    (                  4          ⁢          π          ⁢                      xe2x80x83                    ⁢                      r            xe2x80x2                    ⁢                      r            xe2x80x3                          )            ⁢              P        ⁡                  (                                    c              xe2x80x2                        -                          c              xe2x80x3                                )                                    r        xe2x80x3            -              r        xe2x80x2            
where (4xcfx80rxe2x80x2rxe2x80x3) is the surface area of the capsule, P is the permeability of the wall, rxe2x80x3xe2x88x92rxe2x80x2 is the wall thickness, and cxe2x80x2xe2x88x92cxe2x80x3 is the concentration difference across the wall. The permeability P is the product of the diffusion (D) and partition (K) coefficients of the active ingredient and is largely dependent upon the chemical nature of the wall materials.
Release rates can be appreciably varied by altering the chemical composition and thus the permeability of microcapsule walls. The introduction of disulfide links offers one such approach. Moreover, disulfide linkages are susceptible to cleavage by several agents thereby enabling the possibility of triggered fast release upon demand. Possible triggering agents include base and/or reductive systems.
One aspect of this invention describes microcapsule wall compositions containing disulfide units and providing a semi-permeable barrier. The walls may be made from materials where (1) all the wall forming materials contain sulfur atoms; or (2) some of the wall forming materials contain sulfur atoms and some do not.
Another aspect of this invention describes a process for the introduction of disulfide bonds into microcapsule walls from materials where the disulfide unit (1) is generated during wall formation; or (2) is already present in the starting materials. The first option is preferred when the materials for wall formation are readily available and do not require special preparation in a separate step.